Secure communication method, transmission apparatus and reception apparatus

ABSTRACT

A transmitting apparatus that transmits signals from a plurality of antennas, and can improve the security of communication compared with a conventional system. In this apparatus, an antenna changing section ( 105 ) stores an antenna change pattern in internal memory, and each time a clock signal is input, generates an antenna change signal directing an antenna change in accordance with the antenna change pattern, and outputs this signal to an antenna selection section ( 106 ). Based on the antenna change signal, the antenna selection section ( 106 ) selects two different antennas from among the transmitting antennas ( 107 - 1  through  107 - 3 ) as transmitting antennas of transmit signal A output from a radio section ( 104 - 1 ) and transmit signal B output from a radio section ( 104 - 2 ), and performs radio transmission of transmit signal A and transmit signal B using the selected transmitting antennas.

TECHNICAL FIELD

The present invention relates to a secure communication method thatassures security between a transmitting apparatus and a receivingapparatus when performing radio communication, and a communicationapparatus using that method.

BACKGROUND ART

The widespread use of mobile terminals and so forth in recent years hasbrought demands for improved security between a transmitting apparatusand receiving apparatus when performing radio communication. One exampleof a conventional secure communication method that assures security(Patent Literature 1) will be described using FIG. 1.

In FIG. 1, a transmitting apparatus 10 has a modulation section 11, aradio section 12, and a transmitting antenna 13. Modulation section 11has a transmit digital signal as input, modulates this signal andgenerates a transmit baseband signal, and outputs this signal to radiosection 12. Radio section 12 has the transmit baseband signal as input,up-converts this signal and generates a transmit signal, and transmitsthis signal from transmitting antenna 13 as a radio signal.

On the other hand, a receiving apparatus 50 has a receiving antenna 51,a transmitting antenna 52, a radio section 53, an interference wavegeneration section 54, an interference wave processing section 55, anadding section 56, and a demodulation section 57. Radio section 53 has areceived signal received by receiving antenna 51 as input, down-convertsthis signal and generates a received baseband signal, and outputs thissignal to adding section 56. Radio section 53 also has an interferencewave signal generated by interference wave generation section 54 asinput, up-converts this signal and generates a transmit interferencewave signal, and transmits this signal from transmitting antenna 52 as aradio signal. As a result, a signal in which the transmit signaltransmitted as a radio signal from transmitting antenna 13 and thetransmit interference wave signal transmitted as a radio signal fromtransmitting antenna 52 are added together becomes the received signalreceived by receiving antenna 51.

Interference wave processing section 55 has an interference wave signalgenerated by interference wave generation section 54 as input, performsphase inversion processing and attenuation processing on this signal andgenerates a processed interference wave signal, and outputs this signalto adding section 56.

Adding section 56 has the received baseband signal and processedinterference wave signal as input, and adds these signals together. Bythis means, the interference wave signal is eliminated from the receivedbaseband signal, and an interference wave free signal is generated.Adding section 56 outputs the interference wave free signal todemodulation section 57. Demodulation section 57 has the interferencewave free signal as input, demodulates this signal, and outputs areceived digital signal.

Thus, with the conventional technology, secure communication isperformed by having an interference wave generated by the receivingapparatus and eliminating the interference wave in the receivingapparatus.

-   -   Patent Literature 1: Unexamined Japanese Patent Publication        No.2001-94536

DISCLOSURE OF INVENTION

Problems to be Solved by the Invention

However, in conventional secure communication, since a transmittingapparatus transmits a signal from one transmitting antenna, a radio wavecan be physically received. Therefore, in conventional securecommunication, the security of communication is limited.

It is an object of the present invention to provide a securecommunication method, transmitting apparatus, and receiving apparatusthat enable signals to be transmitted from a plurality of antennas andthe security of communication to be improved compared with theconventional method.

Means for Solving the Problems

According to a secure communication method of the present invention, atransmitting apparatus equipped with a plurality of transmittingantennas performs radio transmission of a signal while switching theaforementioned transmitting antennas based on a predetermined antennachange pattern, and a receiving apparatus performs channel estimationand data demodulation on a signal transmitted as a radio signal from theaforementioned transmitting apparatus using the same change pattern asthe aforementioned transmitting apparatus.

A secure communication method of the present invention has, in atransmitting apparatus that has a plurality of transmitting antennas, astep of inserting a channel estimation symbol in digital data andgenerating a transmit digital signal, a step of up-converting theaforementioned transmit digital signal and generating a transmit signal,a step of selecting antennas in accordance with a predetermined antennachange pattern, and a step of performing radio transmission of theaforementioned plurality of transmit signals using the selectedantennas; and has, in a receiving apparatus that stores the same antennachange pattern as the aforementioned transmitting apparatus, a step ofdown-converting a signal received from the aforementioned transmittingapparatus and generating a received digital signal, a step of separatinga data symbol and channel estimation symbol from the aforementionedreceived digital signal in synchronization with the timing at which theaforementioned transmitting apparatus switches the transmittingantennas, a step of performing channel estimation using theaforementioned separated channel estimation symbol based on theaforementioned antenna change pattern, and a step of demodulating theaforementioned data symbol based on a channel estimate.

According to a secure communication method of the present invention, atransmitting apparatus equipped with a plurality of transmittingantennas performs radio transmission of a transmit signal based on apredetermined signal arrangement pattern, and a receiving apparatusperforms data demodulation on a signal transmitted as a radio signalfrom the aforementioned transmitting apparatus using the same signalarrangement pattern as the aforementioned transmitting apparatus.

A transmitting apparatus of the present invention has a configurationcomprising a plurality of antennas, a frame configuration section thatinserts a channel estimation symbol in digital data and generates aplurality of transmit digital signals, a radio section that up-convertsthe aforementioned transmit digital signals and generates transmitsignals, an antenna changing section that directs an antenna change inaccordance with an antenna change pattern common to the communicatingreceiving apparatus, and an antenna selection section that performsradio transmission of the aforementioned transmit signals usingtransmitting antennas selected in accordance with a directive of theaforementioned antenna change section.

A transmitting apparatus of the present invention has a configurationcomprising a plurality of antennas, a signal generation section thatgenerates a plurality of transmit signals from digital data, a pilotsignal generation section that generates a pilot signal, a signalforming section that arranges the aforementioned plurality of transmitsignals in accordance with a predetermined signal arrangement patternand inserts the aforementioned pilot signal, a signal arranging sectionthat directs the aforementioned signal forming section to perform signalarrangement in accordance with a signal arrangement pattern common tothe communicating receiving apparatus, and a radio section thatup-converts a signal arranged by the aforementioned signal formingsection and generates a transmit signal.

A receiving apparatus of the present invention has a configurationcomprising a radio section that receives and down-converts a signaltransmitted from the aforementioned transmitting apparatus and generatesa received digital signal, a separation section that separates a datasymbol and channel estimation symbol from the aforementioned receiveddigital signal in synchronization with the timing at which theaforementioned transmitting apparatus switches the transmittingantennas, a channel estimation section that performs channel estimationusing the aforementioned separated channel estimation symbol based on anantenna change pattern common to the aforementioned transmittingapparatus, and a signal processing section that demodulates theaforementioned data symbol based on a channel estimate.

A receiving apparatus of the present invention has a configurationcomprising a radio section that receives and down-converts a signaltransmitted from the aforementioned transmitting apparatus and generatesa received baseband signal, a channel estimation section that performschannel estimation using the aforementioned separated channel estimationsymbol based on a signal arrangement pattern common to theaforementioned transmitting apparatus in synchronization with the timingat which the aforementioned transmitting apparatus switches the signalarrangement pattern, and a signal processing section that demodulatesthe aforementioned data symbol based on a signal arrangement patterncommon to the aforementioned transmitting apparatus and a channelestimate.

EFFECTS OF THE INVENTION

According to the present invention, the propagation path of a transmitsignal can be switched by having a transmitting apparatus switch thetransmitting antenna based on a predetermined pattern. On the otherhand, a receiving apparatus can demodulate a received signal byperforming channel estimation using the same pattern as the transmittingapparatus. As a result, even if a radio wave is intercepted by a thirdparty prior to transmitting antenna switching, performing transmittingantenna switching enables subsequent radio wave interception to beprevented and communication security to be improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a drawing showing an example of a conventional securecommunication method;

FIG. 2 is a block diagram showing an example of the configuration of atransmitting apparatus according to Embodiment 1 of the presentinvention;

FIG. 3 is a block diagram showing an example of the configuration of areceiving apparatus according to the above embodiment;

FIG. 4 is a drawing showing an example of frame configurations oftransmit signals according to the above embodiment;

FIG. 5 is a drawing showing propagation channels between a transmittingapparatus and receiving apparatus according to the above embodiment;

FIG. 6 is a drawing showing an example of frame configurations oftransmit signals according to Embodiment 2 of the present invention;

FIG. 7 is a drawing showing an example of frame configurations oftransmit signals according to Embodiment 3 of the present invention;

FIG. 8 is a drawing showing an example of frame configurations oftransmit signals according to Embodiment 4 of the present invention;

FIG. 9 is a drawing showing the overall configuration of a multi-antennacommunication system of Embodiment 5 of the present invention;

FIG. 10 is a block diagram showing the configuration of a transmittingapparatus according to the above embodiment;

FIG. 11 is a block diagram showing the configuration of a receivingapparatus according to the above embodiment;

FIG. 12 is a drawing showing an example of a transmit frameconfiguration according to the above embodiment;

FIG. 13 is a drawing showing an example of a transmit frameconfiguration according to the above embodiment;

FIG. 14 is a drawing showing an example of a transmit frameconfiguration according to the above embodiment;

FIG. 15 is a drawing showing an example of a transmit frameconfiguration according to the above embodiment;

FIG. 16 is a drawing showing an example of a transmit frameconfiguration according to the above embodiment;

FIG. 17 is a drawing explaining a sample implementation of a system thatuses a secure communication method according to the above embodiment;

FIG. 18 is a drawing showing an example of assignment in the frequencydirection of space-time block coded transmit symbols according to theabove embodiment; and

FIG. 19 is a drawing showing a home network system to which the presentinvention can be applied.

MODES FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will now be described in detailwith reference to the accompanying drawings.

EMBODIMENT 1

In Embodiment 1, a case is described in which a transmitting apparatusthat has three antennas switches the transmitting antennas whentransmitting two kinds of signal, and a receiving apparatus that has tworeceiving antennas separates and demodulates received signals from aplurality of channels. In this embodiment, OFDM (Orthogonal FrequencyDivision Multiplexing) is used as an example of a multicarrier method.

First, an example of the configuration of a transmitting apparatusaccording to this embodiment will be described using the block diagramin FIG. 2. In FIG. 2, a transmitting apparatus 100 has frameconfiguration sections 101-1 and 101-2, S/P sections 102-1 and 102-2,IDFT sections 103-1 and 103-2, radio sections 104-1 and 104-2, anantenna changing section 105, an antenna selection section 106, andtransmitting antennas 107-1 through 107-3.

Frame configuration sections 101-1 and 101-2 have transmit digital dataas input, insert channel estimation symbols and guard symbols in thetransmit digital data and generate transmit digital signals, and outputthese signals to S/P sections 102-1 and 102-2.

A channel estimation symbol is a symbol for estimating timesynchronization, frequency synchronization, and distortion due to thetransmission path, that corresponds to a known symbol such as a pilotsymbol, unique word, or preamble, and for which a BPSK (Binary PhaseShift Keying) modulated signal is suitable. A null symbol is usuallyinserted as a guard symbol.

S/P section 102-1 has a transmit digital signal as input, performsserial/parallel conversion processing, and outputs the resulting signalto IDFT section 103-1. S/P section 102-2 has a transmit digital signalas input, performs serial/parallel conversion processing, and outputsthe resulting signal to IDFT section 103-2.

IDFT section 103-1 has a transmit digital signal converted to parallelform as input, performs IDFT conversion processing and generates atransmit baseband signal, and outputs this signal to radio section104-1. IDFT section 103-2 has a transmit digital signal converted toparallel form as input, performs IDFT conversion processing andgenerates a transmit baseband signal, and outputs this signal to radiosection 104-2. IFFT(Inverse Fast Fourier Transform) processing isgenerally used as IDFT conversion processing.

Radio section 104-1 has a transmit baseband signal as input, up-convertsthis signal and generates a transmit signal (hereinafter referred to as“transmit signal A”) , and outputs this signal to antenna selectionsection 106. Radio section 104-2 has a transmit baseband signal asinput, up-converts this signal and generates a transmit signal(hereinafter referred to as “transmit signal B”) , and outputs thissignal to antenna selection section 106.

Antenna changing section 105 stores an antenna change pattern ininternal memory, and each time a clock signal is input, generates anantenna change signal directing an antenna change in accordance with theantenna change pattern, and outputs this signal to antenna selectionsection 106.

Based on the antenna change signal, antenna selection section 106selects two different antennas from among transmitting antennas 107-1through 107-3 as transmit signal A and transmit signal B transmittingantennas, and performs radio transmission of transmit signal A andtransmit signal B using the selected transmitting antennas.

This concludes the description of a sample configuration of atransmitting apparatus according to this embodiment.

Next, an example of the configuration of a receiving apparatus accordingto this embodiment that performs radio communication with thetransmitting apparatus shown in FIG. 2 will be described using the blockdiagram in FIG. 3. In FIG. 3, a receiving apparatus 200 has receivingantennas 201-1 and 201-2, radio sections 202-1 and 202-2, DFT sections203-1 and 203-2, a pattern storage section 204, data separation sections205-1 and 205-2, channel estimation sections 206-1 through 206-4, and asignal processing section 207.

Radio section 202-1 has a received signal received by receiving antenna201-1 as input, down-converts this signal and generates a receivedbaseband signal, and outputs this signal to DFT section 203-1. Radiosection 202-2 has a received signal received by receiving antenna 201-2as input, down-converts this signal and generates a received basebandsignal, and outputs this signal to DFT section 203-2.

DFT section 203-1 has a received baseband signal as input, performs DFTconversion processing on this signal, and outputs the resulting signalto data separation section 205-1. DFT section 203-2 has a receivedbaseband signal as input, performs DFT conversion processing on thissignal, and outputs the resulting signal to data separation section205-2. FFT (Fast Fourier Transform) processing is generally used as DFTconversion processing.

Pattern storage section 204 stores the same antenna change pattern ininternal memory as that stored by antenna changing section 105 shown inFIG. 2, and each time a clock signal is input, generates a transmitpattern information signal indicating the transmitting antennas inaccordance with the antenna change pattern, and outputs this signal todata separation sections 205-1 and 205-2 and signal processing section207.

Based on the transmit pattern information signal, data separationsection 205-1 separates a received baseband signal that has undergoneDFT conversion processing into a transmit signal A channel estimationsymbol, a transmit signal B channel estimation symbol, and a datasymbol, outputs the transmit signal A channel estimation symbol tochannel estimation section 206-1, outputs the transmit signal B channelestimation symbol to channel estimation section 206-2, and outputs thedata symbol to signal processing section 207. Based on the transmitpattern information signal, data separation section 205-2 separates areceived baseband signal that has undergone DFT conversion processinginto a transmit signal A channel estimation symbol, a transmit signal Bchannel estimation symbol, and a data symbol, outputs the transmitsignal A channel estimation symbol to channel estimation section 206-3,outputs the transmit signal B channel estimation symbol to channelestimation section 206-4, and outputs the data symbol to signalprocessing section 207.

Channel estimation section 206-1 has the transmit signal A channelestimation symbol received by receiving antenna 201-1 as input, performstransmit signal A time synchronization, frequency synchronization, andtransmission path distortion estimation processing (hereinafter referredto as “channel estimation”), and outputs a channel estimate indicatingthe processing result to signal processing section 207. Channelestimation section 206-2 has the transmit signal B channel estimationsymbol received by receiving antenna 201-1 as input, performs transmitsignal B channel estimation, and outputs a channel estimate to signalprocessing section 207.

Channel estimation section 206-3 has the transmit signal A channelestimation symbol received by receiving antenna 201-2 as input, performstransmit signal A channel estimation, and outputs a channel estimate tosignal processing section 207. Channel estimation section 206-4 has thetransmit signal B channel estimation symbol received by receivingantenna 201-2 as input, performs transmit signal B channel estimation,and outputs a channel estimate to signal processing section 207.

Signal processing section 207 demodulates data symbols using the channelestimates and transmit pattern information signal, and generates receivedigital data. Examples of the modulation method include a method wherebyinverse matrix computation is performed on a matrix comprising datasymbols using a channel matrix comprising channel estimates, or a methodin which MLD (Maximum Likelihood Detection) is performed.

This concludes the description of a sample configuration of a receivingapparatus according to this embodiment.

A secure communication method according to this embodiment will now bedescribed using above FIG. 2 and FIG. 3, and also FIG. 4 and FIG. 5.FIG. 4 is a drawing showing an example of frame configurations oftransmit signals generated in a transmitting apparatus according to thisembodiment. FIG. 5 is a drawing showing propagation channels between atransmitting apparatus and receiving apparatus according to thisembodiment.

In FIG. 4, a transmit signal A frame is composed of a channel estimationsymbol 301, guard symbol 302, and data symbols 303 in that order, whilea transmit signal B frame is composed of a guard symbol 351, channelestimation symbol 352, and data symbols 353 in that order. The start offrame timing is the same for transmit signal A and transmit signal B,and guard symbols 302 and 351 are inserted so that transmit signal Achannel estimation symbol 301 and transmit signal B channel estimationsymbol 352 do not overlap time-wise. As a result, channel estimationsymbols 301 and 352 are independent time-wise.

The antennas that transmit transmit signal A and transmit signal B areswitched on a frame-by-frame basis in accordance with the antenna changepattern stored in antenna changing section 105. For example, for theframe transmitted between time t1 and time t2, transmit signal A istransmitted from transmitting antenna 107-1, and transmit signal B istransmitted from transmitting antenna 107-2. Then receiving antenna201-1 receives a signal combining transmit signal A subjected to channelfluctuation h11(t) and transmit signal B subjected to channelfluctuation h12 (t) , and receiving antenna 201-2 receives a signalcombining transmit signal A subjected to channel fluctuation h21(t) andtransmit signal B subjected to channel fluctuation h22(t).

Also, for the frame transmitted between time t2 and time t3, transmitsignal A is transmitted from transmitting antenna 107-2, and transmitsignal B is transmitted from transmitting antenna 107-3. Then receivingantenna 201-1 receives a signal combining transmit signal A subjected tochannel fluctuation h12(t) and transmit signal B subjected to channelfluctuation h13(t), and receiving antenna 201-2 receives a signalcombining transmit signal A subjected to channel fluctuation h22(t) andtransmit signal B subjected to channel fluctuation h23(t).

Thereafter, the antennas transmitting the signals are switched in thesame way on a frame-by-frame basis.

Receiving apparatus 200 performs channel estimation using channelestimation symbols in channel estimation sections 206-1 through 206-4.Here, the antenna change pattern of the transmitting apparatus is alsoknown by the receiving apparatus at the start of transmission, andchannel estimation sections 206-1 through 206-4 perform channelestimation anew synchronized with the timing at which the channels onwhich the transmit signals are propagated are switched by using atransmit pattern information signal identical to the antenna changesignal in transmitting apparatus 100.

In channel estimation sections 206-1 through 206-4, channel estimationcan be performed speedily by using channel estimates estimated in thepast for channels after the antennas are switched.

Also, in channel estimation sections 206-1 through 206-4, when a channelestimation symbol is input, the other transmit signal is a guard symbol,and therefore high-quality channel estimation can be performed withlittle interference.

On the other hand, in order to intercept signals transmitted fromtransmitting apparatus 100, a third party must perform channelestimation anew each time transmitting antenna switching is carried outsince the propagation channels vary as a result of transmitting antennaswitching. However, since information indicating transmitting antennaswitching is not contained in transmit signal frames, a third party hasno means of knowing the antenna switching timing or transmit signalpropagation channels. Therefore, a third party cannot intercept signalstransmitted from transmitting apparatus 100.

Thus, in this embodiment, secure communication is implemented by havinga transmitting apparatus that has a plurality of transmitting antennastransmit signals while switching the transmitting antennas based on apredetermined pattern, and having a receiving apparatus perform channelestimation using the same pattern as the transmitting apparatus,synchronized with the timing at which the transmitting antennas of thetransmitting apparatus are switched.

In this embodiment, a case has been described by way of example in whichthe number of transmitting antennas is three and the number of receivingantennas is two, but the present invention is not limited to thisconfiguration, and may also be configured with three or moretransmitting antennas and two or more receiving antennas.

In this embodiment, OFDM is used as an example of a multicarrier method,but the present invention is not limited to this, and can be similarlyimplemented with an OFDM method that uses a spread spectrumcommunication method (DS-CDMA (Direct Spread-Code Division MultipleAccess), FH (Frequency Hopping)-CDMA, UWB (Ultra Wide Band), etc.), or amulticarrier method other than OFDM.

In this embodiment, a case has been described in which the transmittingantennas are switched on a frame-by-frame basis, but the presentinvention is not limited to this, and a security effect can also beobtained by switching the transmitting antennas at a rate of once perplurality of frames, or switching the transmitting antennas at randomintervals.

In this embodiment, channel estimation symbols can be made symbolsinterpolated from past channel estimation symbols—that is, symbols thattrack channel fluctuations over time. For example, in FIG. 4, by using aconfiguration whereby a channel estimation symbol is also transmittedfrom transmitting antenna 107-3 in addition to data symbols beingtransmitted from transmitting antennas 107-1 and 107-2 in the periodfrom time t1 to time t2, symbols that track channel fluctuations can beused from time t2 onward. As a result, since it is difficult to performchannel estimation with only one channel estimation symbol, it becomesmore difficult for a third party to demodulate received signals,enabling security to be further improved.

In this embodiment, a case has been described in which channelestimation symbols and guard symbols are placed before data symbols, asshown in FIG. 4, but the present invention is not limited to this, andit is also possible to use a configuration in which channel estimationsymbols and guard symbols are placed after data symbols, and datasymbols before these symbols are demodulated, or a configuration inwhich channel estimation symbols and guard symbols are placed among datasymbols, and data symbols before and after these symbols aredemodulated.

EMBODIMENT 2

In the present invention, in the above transmitting apparatusconfiguration in FIG. 2 and receiving apparatus configuration in FIG. 3,configurations can be used whereby the antenna that transmits transmitsignal A (107-1) is fixed, and only the antennas that transmit transmitsignal B (107-2 and 107-3) are switched.

To compare this case with the frame configurations in FIG. 4, althoughsecurity falls inasmuch as the transmit signal A propagation channel isnot switched, an advantage is that transmission/reception processing issimplified.

EMBODIMENT 3

In the present invention, with the above transmitting apparatusconfiguration in FIG. 2 and receiving apparatus configuration in FIG. 3,the digital signal encryption pattern can also be changed every antennaswitching cycle (the period until the antenna switching patterncompletes a cycle and returns to its original form), as shown in FIG. 7.FIG. 7 shows a case in which the period from time t1 to time t4 is anantenna switching cycle, with encryption pattern 1 being used for datasymbols transmitted between time t1 and time t4, and encryption pattern2 being used for data symbols transmitted from time t4 onward. Possibledata symbol encryption pattern changes include cases in which thepattern of interleaving or scrambling data is changed, the errorcorrection code (Reed-Solomon code, convolutional code, turbo code, LDPC(Low Density Parity Check) code, etc.) is changed, or the public keyencryption method (RSA, etc.) or private key encryption method (DES,etc.) is changed. For example, even in a case where a signal isintercepted from time t1 to time t4, and it becomes unnecessary toperform channel estimation anew from time t1 onward because channelfluctuation is small from time t1 to time t4, enabling interception tobe easily carried out in time t4, interception can be made difficult bytransmitting with a different encryption pattern from time t4 onward,enabling security to be attained.

EMBODIMENT 4

In the present invention, with the above transmitting apparatusconfiguration in FIG. 2 and receiving apparatus configuration in FIG. 3,antenna switching can also be performed based on the number of transmitpackets, as shown in FIG. 8. Furthermore, the configuration shown inFIG. 8 is possible with various parameters indicating quantity ratherthan time, such as the number of transmit data bits, or the number oftransmissions of packets having priority with regard to packetassignment in transmission (such as the number of transmissions ofpackets that it is wished to transmit without fail in one go, withoutretransmitting). The transmitting antennas may also be switched atrandom intervals for the number of transmit packet bits, number oftransmit packets, or number of packets having priority.

EMBODIMENT 5

In Embodiment 5, a method is described whereby secure communication isperformed using a space-time block code.

FIG. 9 is a drawing showing the overall configuration of a multi-antennacommunication system 800 according to this embodiment. In multi-antennacommunication system 800, a transmitting apparatus 801 has four antennas802-1 through 802-4, and signals are transmitted simultaneously fromeach of antennas 802-1 through 802-4. A receiving apparatus 851 receivessignals simultaneously transmitted from antennas 802-1 through 802-4 bymeans of an antenna 852. A signal transmitted from antenna 802-1 issubjected to channel fluctuation h1(t) and is received by antenna 852,and similarly, signals transmitted from antennas 802-2, 802-3, and 802-4are subjected to channel fluctuations h2(t), h3 (t) , and h4 (t)respectively and are received by antenna 852. In the followingdescription, it is assumed that there is no propagation path (channel)time fluctuation within the time in which space-time block coded signalsare received.

FIG. 10 is a block diagram showing the configuration of a transmittingapparatus 801 according to this embodiment. In FIG. 10, transmittingapparatus 801 is mainly composed of a data splitting section 901,modulation sections 902-1 through 902-3, a pilot signal generationsection 903, a frame configuration signal generation section 904, asignal forming section 905, spreading sections 906-1 through 906-4,radio sections 907-1 through 907-4, and antennas 802-1 through 802-4.

Data splitting section 901 splits transmit data, and outputs the splittransmit data to modulation sections 902-1, 902-2, and 902-3.

Modulation section 902-1 executes digital modulation processing on thetransmit data, and outputs an obtained transmit symbol S1 to signalforming section 905. In the case of QPSK, for example, one transmitsymbol S1 is obtained from 2-bit transmit data. Similarly, modulationsections 902-2 and 902-3 execute digital modulation processing on theirrespective transmit data, and output an obtained transmit symbols S2 andS3 to signal forming section 905.

Pilot signal generation section 903 generates a pilot signal, andoutputs this signal to signal forming section 905.

Frame configuration signal generation section 904 stores a signalarrangement pattern in internal memory, and each time a clock signal isinput, generates a signal arrangement change signal directing a changeof the signal arrangement in accordance with the signal arrangementpattern, and outputs this signal to signal forming section 905.

Signal forming section 905 forms space-time block coded signals usingtransmit symbols S1, S2, and S3, inserts a pilot signal periodically,and outputs the space-time block coded signals and pilot signal tospreading sections 906-1 through 906-4. Signal forming section 905changes the signal arrangement pattern of the space-time block codedsignals each time a signal arrangement change signal is input. Aspecific example of a space-time block coded signal is described laterherein.

Spreading sections 906-1 through 906-4 multiply the respectivespace-time block coded signals by a spreading code and output thepost-spreading signals to radio sections 907-1 through 907-4.

Radio sections 907-1 through 907-4 execute predetermined radioprocessing such as frequency conversion on the output signals ofspreading sections 906-1 through 906-4, and supply the radio transmitsignals thus obtained to antennas 802-1 through 802-4 respectively.

FIG. 11 is a block diagram showing the configuration of receivingapparatus 851 according to this embodiment. In FIG. 11, receivingapparatus 851 is mainly composed of antenna 852, a radio section 1001, adespreading section 1002, channel estimation sections 1003-1 through1003-4, a synchronization section 1004, a frame configuration signalstorage section 1005, and a demodulation section 1006, and receivesspace-time block coded signals transmitted from transmitting apparatus801 in FIG. 9.

Radio section 1001 executes predetermined radio reception processingsuch as frequency conversion on the signals received by antenna 852, andoutputs the received baseband signal thus obtained to despreadingsection 1002. Despreading section 1002 despreads the received basebandsignal, and outputs the post-despreading received baseband signal tochannel estimation sections 1003-1 through 1003-4, synchronizationsection 1004, frame configuration signal storage section 1005, anddemodulation section 1006.

Channel estimation section 1003-1 finds channel fluctuation h1 betweenantenna 802-1 and antenna 852 based on the pilot symbol contained in thesignal transmitted from antenna 802-1, and outputs this to demodulationsection 1006. Similarly, channel estimation sections 1003-2, 1003-3, and1003-4 find channel fluctuations h2, h3, and h4 respectively, and outputthese to demodulation section 1006.

Synchronization section 1004 synchronizes the signals transmitted fromantennas 802-1, 802-2, 802-3, and 802-4 based on the pilot symbolscontained in the received signals, and outputs a timing signal formodulation timing synchronization in the demodulation section todemodulation section 1006.

Frame configuration signal storage section 1005 stores in internalmemory a signal arrangement pattern identical to that stored in frameconfiguration signal generation section 904 shown in transmittingapparatus 10, and each time a received baseband signal is input,generates a signal arrangement change signal directing a change ofsignal arrangement in accordance with the signal arrangement pattern,and outputs this signal to demodulation section 1006.

Demodulation section 1006 has channel fluctuations h1, h2, h3, and h4,received baseband signals (received baseband signals corresponding totimes i, i+1, i+2, and i+3 on the transmitting side being defined asR(t), R(t+1), R(t+2), and R(t+3)), a timing signal, and a signalarrangement change signal as input, performs inverse matrix computationcorresponding to the space-time block coded transmit signal matrix, forexample, based on the signal arrangement change signal, findsdemodulation transmit signals S1, S2, and S3, demodulates these S1 , S2,and S3 signals, and outputs receive digital data.

FIG. 12 is a drawing showing an example of a transmit frameconfiguration according to this embodiment. Shaded areas in the figureindicate pilot symbols, and unshaded areas indicate null symbols (nosignal) . Pilot symbols are transmitted from 802-1 at time i-4, from802-2 at time i-3, from 802-3 at time i-2, and from 802-4 at time i-1.

Then, at times i through i+3, a transmit signal matrix 1110 comprisingtransmit signal column vectors 1101 through 1104 and transmit signal rowvectors 1105 through 1108 is transmitted. The space-time block codingmethod shown in FIG. 12 is the method shown in “Space-Time Block Codingfor Wireless Communications: Performance Results” IEEE JOURNAL ONSELECTED AREAS IN COMMUNICATIONS, pp 451-460, vol.17, no.3, Mar. 1999.An asterisk (*) indicates a complex conjugate. Generally, whenspace-time block coding is executed, transmit signal column vectors (forexample, time i transmit signal column vector 1101 and time i+1 transmitsignal column vector 1102) have an orthogonal relationship, andmaximal-ratio combining of transmit signals can be performedirrespective of channel fluctuation in signal separation on thereceiving side, enabling large diversity gain to be obtained. As aresult, reception quality improves. In addition to the above-describedmethod, known methods of performing maximal-ratio combining on thereceiving side include the use of a correlation matrix. In the presentinvention there are no restrictions on the maximal-ratio combiningmethod.

At this time, transmit symbol S1, S2, and S3 demodulated transmitsymbols S1′, S2′, and S3′ are expressed by equations (1) through (3)below. $\begin{matrix}\left( {{Equation}\quad 1} \right) & \quad \\{S_{1}^{\prime} = \left( {{{R(t)}{h_{1}^{*}\left( {t + 1} \right)}*h_{2}} + \frac{\left( {{R\left( {t + 3} \right)} - {R\left( {t + 2} \right)}} \right)\left( {h_{3}^{*} - h_{4}^{*}} \right)}{2} - \frac{\left( {{R\left( {t + 2} \right)} + {R\left( {t + 3} \right)}} \right)*\left( {h_{3} + h_{4}} \right)}{2}} \right)} & (1) \\{S_{2}^{\prime} = \left( {{{R(t)}h_{2}^{*}} - {{R\left( {t + 1} \right)}*h_{1}} + \frac{\left( {{R\left( {t + 3} \right)} + {R\left( {t + 2} \right)}} \right)\left( {h_{3}^{*} - h_{4}^{*}} \right)}{2} - \frac{\left( {{- {R\left( {t + 2} \right)}} + {R\left( {t + 3} \right)}} \right)*\left( {h_{3} + h_{4}} \right)}{2}} \right)} & (2) \\{S_{3}^{\prime} = \left( {{\frac{\left. {\left( {{R(t)} + {R\left( {t + 1} \right)}} \right)h_{3}^{*}} \right)}{\sqrt{2}}\frac{\left. {\left( {{R(t)} - {R\left( {t + 1} \right)}} \right)h_{4}^{*}} \right)}{\sqrt{2}}} + \frac{{R\left( {t + 2} \right)}*\left( {h_{1} + h_{2}} \right)}{\sqrt{2}} + \frac{{R\left( {t + 3} \right)}*\left( {h_{1} - h_{2}} \right)}{\sqrt{2}}} \right)} & (3)\end{matrix}$

FIG. 13 is a drawing showing a transmit frame configuration when thetransmit signal column vector transmission order is changed around withrespect to the transmit frame configuration shown in FIG. 12, so thattransmit signal column vector 1104 is transmitted at time i, transmitsignal column vector 1101 is transmitted at time i+1, transmit signalcolumn vector 1102 is transmitted at time i+2, and transmit signalcolumn vector 1103 is transmitted at time i+3.

FIG. 14 is a drawing showing a transmit frame configuration when thetransmit signal row vectors assigned to antennas 802-1 through 802-4 arechanged around with respect to the transmit frame configuration shown inFIG. 12, so that transmit signal row vector 1108 is transmitted byantenna 802-1, transmit signal row vector 1105 is transmitted by antenna802-2, transmit signal row vector 1106 is transmitted by antenna 802-3,and transmit signal row vector 1107 is transmitted by antenna 802-4.

Here, in transmitting apparatus 801, even if the transmit frameconfiguration is changed from that in FIG. 12 to that in FIG. 13 or FIG.14, by having frame configuration signal generation section 904 oftransmitting apparatus 801 and frame configuration signal storagesection 1005 of receiving apparatus 851 use the same signal arrangementpattern, maximal-ratio combining of the transmit signals can beperformed, large diversity gain obtained, and reception quality improvedin receiving apparatus 851 after the changes in FIG. 13 or FIG. 14 inthe same way as in the case of FIG. 12.

If a transmit frame configuration is erroneously demodulated on thereceiving side (for example, if a transmit signal transmitted using aFIG. 13 frame configuration is erroneously demodulated as a FIG. 12frame configuration) , the orthogonality of the transmit signal matrixwill be lost, and reception quality will degrade significantly.Considering this from the standpoint of secure communication, there is alarge difference in reception quality between an intended recipient (arecipient who knows the correct transmit frame configuration) and anintercepting party (a recipient who does not know the correct transmitframe configuration), and highly secure communication is possible.

Transmit symbols (16 in FIG. 12) making up a transmit signal matrix(such as 1110 in FIG. 12) that forms space-time block codes are notrestricted as to their placement positions in that transmit signalmatrix, and can be assigned arbitrary within a two-dimensional matrix(time direction/antenna direction). Regarding the number of patterns atthis time, a symbol P is used that indicates the number of patterns whenk items out of n items are aligned, and there are nPk ways (in the caseof FIG. 16, 16P16 (16×15× . . . ×2×1) ways). Unlike a one-dimensionalmatrix such as an M matrix used as a spreading code in spread spectrumcommunication, an extremely large number of patterns can be used, makinghighly secure communication possible. Also, as shown in this embodiment,when space-time block codes are used, a further advantage is that securecommunication is possible without lowering the transfer rate.

The space-time block coding method is not limited to that describedabove, and methods include that shown in “A Quasi-orthogonal Space-TimeBlock Code” IEEE TRANSACTIONS ON COMMUNICATIONS, pp 1-4, vol. 49, no. 1,JANUARY 2001, for example. This method is referred to as aquasi-orthogonal space-time block coding method, and an example of atransmit frame configuration when this method is used is shown in FIG.15. With this method, the transmit signal column vectors of a transmitsignal matrix are configured as partially orthogonal (and the sameapplies to the row vectors), and separation cannot be performed on asymbol-by-symbol basis on the receiving side, with the result thatreceiving-side processing is more complicated, and diversity gainsmaller, than in the case of the orthogonal space-time block codingshown in previously cited “Space-Time Block Coding for WirelessCommunications: Performance Results” IEEE JOURNAL ON SELECTED AREAS INCOMMUNICATIONS, pp 451-460, vol. 17, no. 3, March 1999 (FIG. 12, etc.).However, an advantage of quasi-orthogonal space-time block coding isthat a higher transfer rate is possible than with orthogonal space-timeblock coding. This use of a space-time block code different from anorthogonal space-time block code, such as the above-describedquasi-orthogonal space-time block code, increases the code selectionpatterns and makes more highly secure communication possible.

In the above description, a secure communication method that uses atransmit signal matrix has been described. Below, a secure communicationmethod that uses pilot symbols will be described.

FIG. 16 is a drawing showing a configuration in which the pilot symbolconfiguration has been changed with respect to the transmit frameconfiguration in FIG. 12. When this configuration is used, also, if thispilot symbol configuration is shared by transmitting apparatus 801 andreceiving apparatus 851 in advance, it is possible to perform estimationof channel fluctuations h1, h2, h3, and h4 and demodulate transmitsymbols S1, S2, and S3. However, a receiving apparatus that cannot sharethe pilot symbol configuration shown in FIG. 16 with transmittingapparatus 801, while able to perform pilot symbol estimation, does notknow which of those estimates apply to which of channel fluctuations h1,h2, h3, and h4. It is therefore difficult for such a receiving apparatusto demodulate transmit symbols correctly. Assignment of these pilotsymbols in the time direction and antenna direction can also be thoughtof as assignment within a two-dimensional matrix, and since manypatterns can be used, highly secure communication is possible. Also,when space-time block coded symbols are used as pilot symbols, channelfluctuation estimation can be performed with a high degree of precision,and highly secure communication is possible.

A sample implementation of a system that uses a secure communicationmethod of this embodiment is described below using FIG. 17. FIG. 17shows a sender 1601 capable of communicating using the securecommunication method shown in this embodiment, an intended recipient1602 with whom sender 1601 wishes to communicate, and an interceptingparty 1603 attempting to intercept communications between sender 1601and intended recipient 1602.

Channel fluctuation 1604 between sender 1601 and intended recipient 1602is assumed to be shared between these two parties (for example, in thecase of a TDD (Time Division Duplex) system that uses the same frequencyfor the uplink and downlink, sharing is possible since channelfluctuation can be regarded as identical for the uplink and downlink).

At this time, based on channel fluctuation 1604, sender 1601 transmitswhile controlling the transmit power so as to result in the minimumnecessary CNR (Carrier-to-Noise Ratio) for intended recipient 1602. Bythis means, intended recipient 1602 is able to obtain the necessarydata. However, even if channel fluctuation 1605 between sender 1601 andintercepting party 1603 is a value that can be regarded as the same aschannel fluctuation 1604, intercepting party 1603 cannot demodulate thespace-time block code correctly without knowing the transmit frameconfiguration, and therefore suffers greatly degraded reception quality,and has extreme difficulty in decoding the data that intended recipient1602 needs.

As explained above, using a system of this embodiment makes possiblehighly secure communication between the intended parties. At this time,the present invention can be similarly implemented if a configuration isused whereby intended recipient 1602 transmits an RSSI (Received SignalStrength Indicator) value, for example, to sender 1601 as a valueindicating reception quality.

The above-described embodiments are also effective with UWB (Ultra WideBand) communication. When UWB communication is performed in theseembodiments, configurations are used in which the radio sections areomitted from transmitting apparatus 801 in FIG. 10 and receivingapparatus 851 in FIG. 11. In UWB communication, transmission andreception is performed by spreading a signal over an extremely widefrequency band of around 1 GHz, and signals transmitted in therespective frequency bands have power of the level of noise. Therefore,unless an intercepting party shares the pattern with the transmitter,only extremely low power can be used, and interception is extremelydifficult. Also, the fact that the size relationship of the receivedsignal power from each antenna is also low is another point facilitatingthe execution of secure communication of this embodiment.

In the above embodiments, the descriptions have assumed that the antennachange pattern, space-time block coding transmission row vector changepattern, and so forth, are shared in advance, but the probability ofinterception can also be lowered in a case where an intercepting partydemodulates all patterns on a round-robin basis by making a patternchange based on an RSSI value, for example. By making this patternchange, secure communication of this embodiment is made more highlysecure. In this case, in transmitting apparatus 801, the signalarrangement pattern is changed by inputting signal arrangement patterninformation to frame configuration signal generation section 904. Inreceiving apparatus 851, a received baseband signal is input to frameconfiguration signal storage section 1005 and the signal pointarrangement is changed, and demodulated transmission symbols areobtained using a frame configuration signal and timing information.

By transmitting information that is not wished to be, or must not be,divulged to any but the intended communicating party on any account—suchas the aforementioned antenna change pattern, the aforementionedspace-time block coding transmission row vector change pattern, anencryption key in communication in which encryption is performed, and soforth—using the secure communication method described in thisembodiment, and transmitting other data without performing the securecommunication of this embodiment, highly secure communication is madepossible while keeping degradation of data transmission efficiencysmall.

In the method shown in this embodiment, space-time block coded transmitsymbols are assigned in the time direction and antenna direction byrearranging the transmit signal row vectors or column vectors, but thisembodiment is not limited to this assignment method, and can besimilarly implemented by spreading and assigning space-time block codedtransmit symbols so as to be randomly interleaved. It is also possiblefor assignment to be performed in the frequency direction. FIG. 18 is adrawing showing an example of assignment of space-time block codedtransmit symbols in the frequency direction, illustrating a case inwhich transmission is performed using OFDM (Orthogonal FrequencyDivision Multiplexing) modulation. The basic transmitter and receiverconfigurations when using OFDM modulation are as shown in FIG. 2 andFIG. 3, and are therefore not described in detail here. A system usingOFDM modulation is configured by omitting the spreading sections in FIG.10 and inserting S/P sections and IFFT sections, and omitting thedespreading sections in FIG. 11 and inserting P/S sections and FFTsections.

FIG. 18 is a drawing showing an example of the transmit frameconfiguration when an OFDM modulated signal (using four subcarriers) istransmitted from transmitting apparatus 801 shown in FIG. 9. Transmitsignal column vector 1101 allocated to antennas 802-1 through 802-4 inFIG. 12 is arranged in subcarriers 1 through 4 of the OFDM modulatedsignal transmitted by antenna 802-1, and similarly, transmit signalcolumn vectors 1102, 1103, and 1104 are arranged in the subcarriers ofthe OFDM modulated signals transmitted by antenna 802-2, 802-3, and802-4, respectively. When using an OFDM modulated signal with this kindof arrangement, arrangement is possible in the time direction andfrequency direction within the OFDM modulated signal (although onlyarrangement in the frequency direction is shown in FIG. 18, it is clearthat arrangement can also be performed in the time direction), and atransmit frame configuration using a three-dimensional matrix ispossible by adding antenna direction assignment to these two, enablingmore patterns to be used than in the case of a one-dimensional matrix ortwo-dimensional matrix configuration. Thus, more highly securecommunication is possible.

In the above description the number of subcarriers has been assumed tobe four, but this is just one example, and in general, communication canbe made more secure the greater the number of subcarriers used. The samealso applies to the number of antennas, with the present invention notbeing limited to four transmitting antennas and one receiving antenna,but able to be similarly implemented using, for example, twotransmitting antennas and two receiving antennas. As the receptionquality can be improved by maximal-ratio combining of the transmitsymbols demodulated via the two receiving antennas, the difference inreception quality between the intended recipient and an interceptingparty is increased, and more highly secure communication can beperformed.

In this embodiment, when spreading is not performed, configurations areused in which the spreading sections in FIG. 10 and the despreadingsections in FIG. 11 are omitted.

OTHER EMBODIMENTS

A home network system will now be described as an example of the use ofa system of the present invention.

In recent years, digital playback and recording apparatuses using a harddisk (HD) , DVD-ROM, or the like as a storage medium have becomeincreasingly popular. A digital playback and recording apparatus doesnot suffer from deterioration of information, and can implementfunctions that are difficult or impossible with a conventional VCR, suchas fast skipping to a particular playback point, and allowing a user totemporarily halt a program being broadcast and then continue viewingseveral minutes later. In addition, connection to the Internet allowsdigital playback and recording apparatus operations to be carried outvia a network, including downloading of the latest EPG (ElectronicProgram Guide) and operating the apparatus from outside the home.

At the same time, ADSL, FTTH, and suchlike broadband circuits arebecoming increasingly widely used for wireless LAN systems in the home.Furthermore, home network systems that transmit television and videoimages within the home using a wireless system are also starting tobecome popular. An example of such a system is shown in FIG. 19. Areceived signal received by a receiving antenna 1801 is stored in a homeserver 1802. Also, a received signal distributed from a network 1803 bymeans of TCP/IP transmission or the like via an ADSL, FTTH, or similarbroadband circuit is stored in home server 1802. Here, home server 1802includes a digital playback and recording apparatus with a hard disk orthe like as a storage medium.

Home server 1802 performs signal processing of a received signalreceived from receiving antenna 1801 or network 1803, storage mediumvideo, voice, data, and so forth, and performs radio transmissionthereof to a PC (Personal Computer) 1804 and TV (Television) 1805 in thehome. PC 1804 and TV 1805 each have a receiving antenna, receive asignal transmitted from home server 1802, and obtain and display or playvideo, voice, and/or data. This kind of wireless system offers suchadvantages as the ability to use PCs and TVs at various locations in thehome and the fact that troublesome wiring is unnecessary, and isexpected to become increasingly popular in the future.

In comparison with a system that uses a single antenna for transmissionand reception, a system that transmits and receives a plurality ofsignals using a plurality of antennas such as the system of the presentinvention theoretically enables channel capacity to be increased in thesame frequency band, and has been the subject of a great deal ofresearch. Increased channel capacity is extremely useful in anabove-described wireless system in which high-speed, large-volumetransmission is performed.

In the above-described wireless system, radio waves are generally notonly propagated to a person or object desiring reception, but also tosurrounding persons and objects. In the case shown in FIG. 19, forexample, a transmit signal transmitted to PC 1804 and TV 1805 of acertain household from home server 1802 also reaches a PC 1806 and TV1807 of neighboring households. In this case, the transmit signal fromthe home server may cause interference in PC 1806 and TV 1807, and thetransmit signal from the home server may also be interceptedintentionally. When a signal is intercepted intentionally, inparticular, various problems can be envisaged, such as a privacy problemof knowing which program is being viewed in the case of a broadcastsignal, a problem of a person with no contract being able to receive afee-paying broadcast free-of-charge, and a problem of digital contentbeing reused.

With regard to intentional interception, the fragility of security incurrent wireless LAN systems is one well-known and serious problem.Although access control is implemented by means of an ID or passwordwhen an access point such as a wireless LAN modem is accessed in awireless LAN system, many cases have been reported of an ID being easilyguessed or a password being cracked within a few days or so, and similarproblems may also occur when access control is implemented by means ofIDs, passwords, or the like in a system that uses a plurality oftransmitting and receiving antennas. A different kind of securityimplementation method from those used heretofore is thus desirable.

When a plurality of antennas are used, if those antennas are always thesame the channel on which a signal is propagated cannot be switched, andtherefore once a signal has been intercepted by a third party there is apossibility that interception will continue. However, if thetransmitting antennas are switched, the channel on which a signal ispropagated will also be different after the switchover, and securityagainst transmit signal interception can be improved by performing thisswitching continuously.

The present application is based on Japanese Patent ApplicationNo.2003-318809 filed on Sep. 10, 2003, and Japanese Patent ApplicationNo.2004-258919 filed on Sep. 6, 2004, the entire content of which isexpressly incorporated herein by reference.

INDUSTRIAL APPLICABILITY

The present invention is suitable for use in a base station apparatusand communication terminal apparatus of a radio communication system inwhich secure communication is performed.

1. A secure communication method, wherein: a transmitting apparatusequipped with a plurality of transmitting antennas performs radiotransmission of a signal while switching said transmitting antennasbased on a predetermined antenna change pattern; and a receivingapparatus performs channel estimation and data demodulation on a signaltransmitted as a radio signal from said transmitting apparatus using thesame change pattern as said transmitting apparatus.
 2. A securecommunication method; comprising in a transmitting apparatus that has aplurality of transmitting antennas: a step of inserting a channelestimation symbol in digital data and generating a transmit digitalsignal; a step of up-converting said transmit digital signal andgenerating a transmit signal; a step of selecting antennas in accordancewith a predetermined antenna change pattern; and a step of performingradio transmission of said plurality of transmit signals using selectedantennas; and comprising in a receiving apparatus that stores the sameantenna change pattern as said transmitting apparatus: a step ofdown-converting a signal received from said transmitting apparatus andgenerating a received digital signal; a step of separating a data symboland channel estimation symbol from said received digital signal insynchronization with timing at which said transmitting apparatusswitches transmitting antennas; a step of performing channel estimationusing said separated channel estimation symbol based on said antennachange pattern; and a step of demodulating said data symbol based on achannel estimate.
 3. A secure communication method, wherein: atransmitting apparatus equipped with a plurality of transmittingantennas performs radio transmission of a transmit signal based on apredetermined signal arrangement pattern; and a receiving apparatusperforms data demodulation on a signal transmitted as a radio signalfrom said transmitting apparatus using the same signal arrangementpattern as said transmitting apparatus.
 4. The secure communicationmethod according to claim 3, wherein said signal arrangement pattern isa pilot signal matrix signal arrangement pattern.
 5. The securecommunication method according to claim 3, wherein said signalarrangement pattern is a transmit signal matrix signal arrangementpattern.
 6. A transmitting apparatus, comprising: a plurality ofantennas; a frame configuration section that inserts a channelestimation symbol in digital data and generates a plurality of transmitdigital signals a radio section that up-converts said transmit digitalsignals and generates transmit signals; an antenna changing section thatdirects an antenna change in accordance with an antenna change patterncommon to a communicating receiving apparatus; and an antenna selectionsection that performs radio transmission of said transmit signals usingtransmitting antennas selected in accordance with a directive of saidantenna change section.
 7. A transmitting apparatus comprising: aplurality of antennas; a signal generation section that generates aplurality of transmit signals from digital data; a pilot signalgeneration section that generates a pilot signal; a signal formingsection that arranges said plurality of transmit signals in accordancewith a predetermined signal arrangement pattern and inserts said pilotsignal; a signal arranging section that directs said signal formingsection to perform signal arrangement in accordance with a signalarrangement pattern common to a communicating receiving apparatus; an aradio section that up-converts a signal arranged by said signal formingsection and generates a transmit signal.
 8. A receiving apparatuscomprising: a radio section that receives and down-converts a signaltransmitted from the transmitting apparatus according to claim 6 andgenerates a received digital signal; a separation section that separatesa data symbol and channel estimation symbol from said received digitalsignal in synchronization with timing at which said transmittingapparatus switches transmitting antennas; a channel estimation sectionthat performs channel estimation using said separated channel estimationsymbol based on an antenna change pattern common to said transmittingapparatus; and a signal processing section that demodulates said datasymbol based on a channel estimate.
 9. A receiving apparatus comprising:a radio section that receives and down-converts a signal transmittedfrom the transmitting apparatus according to claim 7 and generates areceived baseband signal; a channel estimation section that performschannel estimation using said separated channel estimation symbol basedon a signal arrangement pattern common to said transmitting apparatus insynchronization with timing at which said transmitting apparatusswitches a signal arrangement pattern; and a signal processing sectionthat demodulates said data symbol based on a signal arrangement patterncommon to said transmitting apparatus and a channel estimate.